By default, you will play with a ruler that eight units long, and one unit is a 1x4 click hinge brick or 5 studs in length.
Combining eight hinge bricks gives you a ruler that can measure around corners. Especially useful to plot paths through or around cover and obstacles.
The game area should measure between 4 and 6 ruler lengths diagonally and circular; or square. With hinge bricks, 4 lengths is the equivalent to about 45"/115cm. 6 lengths is the equivalent to about 60"/150cm.
As a general rule when measuring between two targets, measure between the closest points of them. Barrel to body part, blade to arm, back to cover, etc.